The use of semi-permeable membranes to accomplish various fluid exchange, removal or diffusion functions has been known for some time, e.g., in the fields of salt-water purification, dialysis and gas exchange. More specifically there has been some application of such technology to blood oxygenators, e.g., U.S. Pat. No. 3,332,746, the disclosure of which is incorporated by reference herein, which function to maintain the desired oxygen-carbon dioxide balance in blood when the cardiorespiratory system is not totally capable of doing so. However, the most widely used blood oxygenators at the current time are those of the bubbler type in which oxygen is introduced into blood through small diameter orifices such that bubbles are formed in the blood. Exemplary disclosures of bubbler type blood oxygenators may be found in U.S. Pat. Nos. 3,265,883; 3,468,631; 3,488,158, and 3,578,411, the disclosures of which are incorporated by reference herein. Theoretically, membrane oxygenators would possess certain advantages as compared to bubbler type or disc type oxygenators including reduction of the risk of trauma to the blood, but the membrane oxygenators presently available for use do not enjoy widespread use because of inefficient diffusion transport rates, relatively large blood priming volumes and high cost due, among other things, to difficulty of assembly.
In order to accomplish gas transfer through a membrane, it is necessary to overcome the resistance created by two principal causes, namely, the membrane itself and the liquid layer on the surface of the membrane. In a blood oxygenator, the blood which is to be oxygenated is on one side of the membrane and the oxygen which is to be diffused into the blood is on the other and the membrane is gas pervious but blood impervious. In membrane oxygenators of the type currently being clinically used, it is estimated that about 60% to 85% of the resistance to diffusion of oxygen and carbon dioxide (the latter being removed from the blood) is caused by the blood film which forms on the surface of one side of the membrane. Typically such oxygenators employ silicone rubber alone or in combination with other materials as the membrane material.
Various approaches have been employed in attempts to overcome the boundary layer resistance of the blood to gas diffusion. Among them have been the use of extremely small diameter tubular membrane configurations, on the order of 100.mu. to 300.mu., reduce the size of the blood film. However, several thousand such capillary-size tubes are required to accommodate the necessary blood volume and the small size creates a risk of thrombus formation. A variation of this approach is to place an even smaller diameter tube within a capillary tube such that the blood flows in the annular space between them and, by using pulsatile blood and gas flows, some reduction in boundary layer effects is achieved.
Another capillary flow technique includes the use of flat sheet membranes enveloping plates having capillary grooves therein.
Various attempts to reduce boundary layer effects by inducing turbulent flow to displace the boundary layer include the use of torsionally oscillating toroidal membrane chambers.
Reduction of the thickness of the liquid film has been attempted by the use of sheet membranes over separater plates having capillary furrows extending transversely to or with the direction of blood flow and by the use of pulsatile or steady gas and/or blood flows. Another approach has been the use of a plurality of tubular membranes enclosed in a flexible sleeve which sleeve is pulsed to induce blood blow within the tubular membranes.
The foregoing approaches have required the use of bulky external supporting equipment, achieved only marginal improvement in diffusion or have otherwise been subject to disadvantages such that only a minor proportion of clinical oxygenator use has involved membrane type units. In this regard, the desirability that such oxygenators be of the disposable type should also be noted, since several of the foregoing approaches do not lend themselves to disposability.